The present invention relates to interferometers for detecting and measuring extremely small dimensions or distances, especially of objects that are in relative motion. It is known, for example, that the magnetic transducer element or slider used in recording and reading magnetic information on a spinning disc must "fly" over the surface of the disc at a predetermined height which is extremely small (i.e., 0.4 microns) without rolling or pitching. The interferometer of the present invention enables this height to be measured while the disc is in motion and to determine if, in fact, the slider is pitching or rolling with respect to the disc.
Heterodyne interferometers are well-known and understood, especially in applications where it is desired to determine the shape (i.e., flatness) of a surface such as of a mirror, for example. The basic principle is to produce a monochromatic light beam and to split the beam into two components, each of a different polarization and frequency. One component is directed to and ultimately reflected from the surface being examined; the other interacts similarly with a reference surface. By bringing the reflected beams together, an interference pattern characteristic of the test surface is produced. It will be understood that the variation from point to point of the interference pattern results from the differences of deviations between the two reflecting surfaces. This interference pattern is detected as an optical beat frequency, the phase of which varies in proportion to the variation in optical path length between the test and reference areas of the interferometer. It is customary to convert light from a point in the optical image of the interference pattern into electrical signals and to store these signals for comparison with signals subsequently derived by repeating the process for additional points on the test surface, each such point providing a characteristic phase of the optical beat frequency which will be different for each point if there are deviations in the test surface at each point. The phase difference between points on the test surface being a measure of the extent of the deviations between the various points and thus indicative of the shape (topography) of the surface.
Typical of the interferometers of the prior art are U.S. Pat. Nos. 4,188,122 and 4,340,304 to N.A. Massie et al, and N.A. Massie, respectively. In both patents, the ultimately obtained light beams or images from the reflecting surfaces are combined and collapsed into a single plane of polarization so that they optically interfere and produce an interference pattern which is scanned in a single plane to produce electrical signals representative of the deviations of the test surface relative to the reference surface.
In the interferometer systems of the prior art the topography of the surface under examination is determined by examining each point sequentially. This is permissible since the surface under survey is substantially fixed or immobile or, if in motion or undergoing a change, the rate of change is so slow that for all pratical purposes there is no noticeable change in the surface due to motion or change thereof during the time elapsing between observations. Such a sequential observation technique is acceptable under the "static" circumstances just defined. In a dynamic situation where the surface under examination may be in a state of rapid change, defined as being a rate of change between observation points faster than the elapsed time between observations, the observations from point to point cannot be made sequentially but must be performed substantially simultaneously. This is the situation where it is desired to determine the flying height of a magnetic transducer head as it flies over the surface of the rapidly spinning recording disc. Variations in this flying height occur so rapidly that a point on the head must be observed substantially simultaneously with the observation of a point of the recording disc. The present invention provides means and ways to accomplish such observations with interferometer apparatus.